Apparatus and process for removing volatile coatings from scrap metal

ABSTRACT

Both aluminum scrap covered with a volatile coating and a heated airstream having a low oxygen content pass through a kiln where the airstream causes the coating to volatilize. The temperature of the airstream, where it enters the kiln, is maintained essentially constant, slightly below the melting temperature of the aluminum, and likewise the temperature of the airstream is maintained essentially constant where the airstream leaves the kiln, this being achieved by varying the mass flow of the airstream to compensate for variances in the nature and mass of the aluminum scrap within the kiln. Beyond the kiln the airstream enters an afterburner where it is heated in the presence of sufficient oxygen to effect combustion of the volatile components of the coating. The airstream then passes through a heat exchanger and back to the kiln. The mass flow within the kiln is controlled by diverting some of the airstream from the heat exchanger or by varying the speed of the fan which creates the airstream.

RELATED APPLICATION

This application is a division of application Ser. No. 07/401,795, filedSep. 1, 1989, U.S. Pat. No. 5,059,116, which is a continuation-in-art ofapplication Ser. No. 07/285,637, filed Dec. 16, 1988, now abandoned.

BACKGROUND OF THE INVENTION

This Application relates in general to processing metals and moreparticularly to an apparatus and process for removing volatile coatingsfrom scrap metal.

Because the energy required to melt aluminum metal is considerably lessthan that required to extract aluminum from its ores, much of thealuminum used in manufactured goods derives from aluminum scrap--and oneof the principal sources of aluminum scrap is discarded beverage cans.Because the typical aluminum beverage can has an organic coating,usually a lacquer, on its interior and exterior surfaces, aluminumbeverage cans tend to produce a considerable amount of dross whenintroduced into a melting furnace. In this regard, within the furnacethe coating volatizes and ignites before the can melts, and thecombustion which ensues oxidizes the aluminum, thereby creating thedross which is actually an oxide of aluminum. Processors of aluminumscrap therefore usually subject aluminum beverage cans to a delacqueringoperation before introducing them into a melting furnace. Also, beveragecans may contain residual moisture, perhaps in the form of the beverageitself, and to ensure the safety of those operating the melting furnace,the moisture should be eliminated before the cans enter the furnace.

The typical delacquering process attempts to subject the coated beveragecans to temperatures high enough to volatilize the coating, but not sohigh as to melt the aluminum. Some processes even operate at reducedoxygen levels to lessen the chances of ignition, but all currentprocesses are difficult to control. A slight change in the moisturecontent can reduce the operating temperature to the point that much ofthe coating remains, or an increase in the mass of the cans processedmay cause the system to react such that it produces excessivetemperatures which ignite the coating on some of the cans.

The present invention resides in an apparatus and process whichcirculates heated air of reduced oxygen content past coated aluminumscrap to remove a coating from the aluminum. Even though the nature andmass of the scrap may vary, the rarefied air remains at a substantiallyuniform temperature somewhat below melting temperature of the aluminum,and hence the aluminum experiences little oxidation. By varying the massflow of the rarefied air, the system compensates for variances in thecontent of the scrap and the mass of the scrap.

DESCRIPTION OF THE DRAWINGS

In accompanying drawings which form part of the specification andwherein like numerals and letters refer to like parts wherever theyoccur

FIG. 1 is a schematic view showing the components of a system forremoving volatile coatings from scrap metal;

FIG. 2 is a graph showing the temperature of the air and scrap in thekiln of the system; and

FIG. 3 is a schematic view of a modified system.

DETAILED DESCRIPTION

Referring now the drawings (FIG. 1), a system A for removing coatingsfrom aluminum scrap includes several basic components, namely a kiln 2,a cyclone collector 4, a recirculation fan 6, an incinerator orafterburner 8, a system exhaust 10, and a heat exchanger 12, allarranged within a closed flow path or loop through which an airstreamflows. The air of the airstream encounters the scrap in the kiln 2 whereit elevates the temperature of the scrap to about 1025° F., which isless than the melting temperature of the aluminum, and at thattemperature the organic coatings volatilize and enter the airstreamwhich flows through the kiln 2 in the direction opposite to that atwhich the scrap travels. The air within the kiln 2 has a reduced oxygencontent on the order of 6% to 8%, and at that level the coating does notignite. The scrap should exist in relatively small fragments, crushedbeverage cans being typical. The aluminum of these cans is quite thin,and thus presents a large surface area in comparison to its mass. Whenthe temperature of the coating approaches that at which the aluminummelts, the coating may ignite and oxidize the aluminum, therebycoverting the valuable aluminum into useless dross. Thus, it isimportant to prevent the coating from igniting in the presence of thealuminum. Typical coatings are lacquers which volatilize below themelting temperature of aluminum.

In addition to the foregoing major components, the system A has threecontrol loops 14, 16, and 18 which sense the condition of the airstreamat different locations throughout the system and cooperate to maintainthe temperature of the airstream at the entrance to the kiln 2 at about1125° F. and the oxygen content in the kiln at between 6% and 8%.

Turning now to the kiln 2, it is coupled to an electric motor 22 whichrotates it at between 4 and 8 rev/min. The kiln 2 encloses a cylindricalchamber 24 through which the scrap passes, the scrap being introducedinto the chamber 24 at one end through an air lock 26 and being removedat the opposite end through another air lock 28. Moreover, the kiln 2 isinclined slightly downwardly, with its scrap discharge end being belowits scrap feed end, and it further possesses flights which cause thescrap to tumble as it passes through the kiln 2. The air locks 26 and 28in effect seal the ends of the chamber 24 and thus isolate the chamber24 from the surrounding atmosphere. As a consequence, it is possible tocontrol the oxygen content of air passing through the chamber 24, andnormally that oxygen content is maintained at between 6% and 8% computedon a volume basis. On the other hand, the temperature of the airentering the chamber 24 is maintained substantially constant within alimited range between 1000° F. and 1200° F. and preferably at 1125° F.This air is derived from a duct 30 which opens into the discharge end ofthe chamber 24, that is the end at which the scrap passes into the airlock 28. The air within the chamber 24 never experiences a highertemperature, but instead the scrap and kiln 2 extract heat from the airas the air moves through the chamber 24 toward the feed end. Indeed, inflowing through the chamber 24, the air experiences a substantialdecrease in temperature. The lower temperature air leaves the chamber 24through a duct 32 which is connected to the kiln 2 at the feed end ofits chamber 24. Indeed, the temperature of the air where it leaves thechamber 24 is likewise maintained substantially constant at between 400°F. and 500° F. and preferably at 450° F., this being achieved by varyingthe mass of the air flowing through the chamber 24.

Owing to the reduced oxygen content of the air within the chamber 24 ofthe kiln 2, the coating on the scrap does not ignite within the chamber24, but for the most part volatilizes and enters the airstream, althougha portion may leave as particles which become entrained in theairstream. Indeed, the oxygen content of the air discharged into thechamber 24 from the duct 30 is maintained low enough to prevent ignitionof the coating. The time for a fragment of scrap to pass through thekiln 2, that is from when it passes through the air lock 26 and enterschamber 24 to when it passes out of the chamber 24 at the air lock 28,is about 16-20 minutes. The mass of the air passing through the chamber24 varies, it being dependent on the amount and nature of the scrap, butirrespective of the mass flow, the temperature of the air entering thechamber 24 at the duct 30 remains essentially constant. For example, anincrease in the amount of scrap will trigger a greater volume--anincrease in mass flow--of air flowing through the chamber 24 per unit oftime, but the temperature of the air entering the chamber 24 will remainessentially at 1125° F. The same holds true when the moisture content ofthe scrap increases.

The duct 32, which is connected to the feed end of the kiln 2, leads tothe cyclone collector 4 into which it opens, and thus the air along withthe vapor and the particulates are discharged into the cyclone collector4. Here, by centrifugal force, most of the particulates are removed fromthe airstream and they collect at the bottom of the collector 4, fromwhich they are withdrawn from time to time through an air lock 34.

Leaving the collector 4, the airstream passes into another duct 36 whichleads to the recirculation fan 6--actually to the suction port of thefan 6. The discharge port of the fan 6 is connected to still anotherduct 38 which leads to the afterburner 8. The fan 6, while having avariable speed electric motor 37, normally operates at a constant speed.However, the speed of the motor 37 is gradually reduced during theinterval when the system A is shut down, and this avoids sending anexcessive amount of air into the kiln 2 to remove lacquer fromsubstantially reduced mass of scrap.

The afterburner 8 encloses a combustion chamber 40 and includes a burner42 which is directed into one end of the combustion chamber 40. Theburner 42 operates on a combustible gas, such as natural gas, which isfed to it through a fuel line 44 containing a valve 45. The air forsupporting this combustion is supplied through a combustion air line 46containing a valve 47. Actually the air line 46 leads directly to theburner 42, whereas another air line 48 leads to the combustion chamber40, opening into it slightly downstream from the burner 42. The air line48 also contains a valve 50. In this regard, the air stream, uponleaving the kiln 2, posseses less than the normal proportion of oxygen,on the order of 6% to 8%, and in order to completely consume the vaporsderived from the volitalized coating, additional oxygen must besupplied. That additional oxygen is derived from the air line 48, butsome of that oxygen is converted into carbon dioxide and water vapor asa result of the combustion of the volatile components in the afterburner8. Indeed, the controls within the system A regulate the amount of airsupplied through the air line 48 such that the airstream, where it isdischarged from the afterburner 8, possesses an oxygen content of about6% to 8%. The valve 45 in the fuel line 44 and the valve 47 in the airline 46 are operated by a common motor 52. The valve 50 in the air line48 that leads to the combustion chamber 40 downstream from the burner 42is controlled by another motor 54.

The heated and rarefied air discharged from the afterburner 8 enters yetanother air duct 56 which leads past the system exhaust 10 to the heatexchanger 12. The system exhaust 10 includes in an exhaust duct 58 whichintersects the air duct 56 and leads to a bag house or to theatmosphere. It further includes an exhaust damper 59 and a motor 60which operates the damper 59 to vary the effective cross-sectional sizeof the duct 58, thus controlling the volume of rarefied air that isallowed to escape from the system A between the afterburner 8 and theheat exchanger 12.

The heat exchanger 12 is interposed between air duct 56 that leads awayfrom the afterburner 8 and the air duct 30 that leads into the kiln 2,and as such the airstream which passes into the heat exchanger 12 fromthe duct 56 leaves at a lower temeprature through the duct 30. The heatexchanger 12 contains two flow paths 62 and 64, the former being for theheated airstream and the latter for cooler air which extracts heat fromthe airstream, thus lowering the temperature of the airstream. Thecooler air is derived from a blower 65 which is connected to the flowpath 64 through a supply duct 66. The opposite end of the flow path 64,on the other hand, opens into a discharge duct 68 which is in turnconnected to the air lines 46 and 48 that lead to the afterburner 8.Thus, the air for supporting combustion of natural gas at the burner 42is heated to render the burner 42 more efficient, and the same holdstrue with respect to the air that is supplied directly to the combustionchamber 40 of the afterburner 8 for incinerating the vapors derived fromthe coating. Intersecting the discharge duct 68 is a vent duct 70 whichcontains a damper 72 that is operated by a motor 74. The damper 72, ofcourse, controls the cross-sectional size of the duct 70, which in turncontrols the back pressure on the blower 65 and the amount of coolingair that flows through the flow path 64 of the heat exchanger 12.

Thus, within the flow path 62 of the heat exchanger 12, the rarefiedairstream experiences a reduction in temperature--indeed a reduction tothe temperature best suited for removing the coating from the aluminumscrap in the cylindrical chamber 24 of the kiln 2. The cooling air thatpasses through the flow path 64, on the other hand, undergoes a rise intemperature, and as such more efficiently supports combustion at theafterburner 8.

The first control loop 14 includes a temperature sensor 80 which islocated along the duct 30 close to the location where the duct 30discharges into the kiln 2. It senses the temperature of the airstreamat the entrance to the cylindrical chamber 24 of the kiln 2. The sensor80 is connected to a controller 82 which generates signals that operatethe motor 74 which controls the damper 72 in the vent duct 70. This, inturn, controls the back pressure on the blower 65. The higher the backpressure, the less the volume of air flowing through the path 64 of theheat exchanger; and conversely, the lower the back pressure, the greaterthe volume of air. If the temperature of the airstream decreases at theentrance to the kiln 2, the controller 82 generates a signal whichcauses the motor 74 to close the damper 72, thus reducing the flow ofcooling air through the flow path 64 of the heat exchanger 12 andextracting less heat from the airstream as it passes through the flowpath 62. This increases the temperature of the airstream. Of course, ifthe temperature at the entrance to the kiln 2 increases, the oppositeoccurs. Thus, the first control loop 14 maintains the airstream at anessentially constant temperature where the airstream enters the kiln 2.

The second control loop 16 monitors the temperature of the airdischarged from the cylindrical chamber 24 of the kiln 2, and itspresence insures that an adequate supply of air passes through the kiln2. It includes a temperature sensor 84 that is located along the duct 32which leads from the kiln 2 to the cyclone collector 4 and a controller86 which generates control signals in response to the temperaturessensed by the sensor 84. The controller 86 is connected to and operatesthe motor 60 which controls the exhaust damper 59 of the system exhaust10. Even though the temperature of the airstream remains essentiallyconstant where it enters the cylindrical chamber 24 of the kiln 2, theamount of heat transferred to the scrap in the kiln 2 can vary widelydepending on several factors. Among these factors are the mass orquantity of scrap and the moisture content of the scrap within the kiln2. For example, a greater amount of scrap or moisture will extract moreheat from the airstream and thus cause a greater decrease in thetemperature of the airstream as it passes through the kiln 2, assumingof course that volume of the airflow--or more accurately the massflow--remains constant. However, the temperature of the airstream shouldremain relatively high throughout the chamber of the kiln 2 in order tocompletely volatilize the coating on the scrap during the limitedresidence time for the scrap within the chamber 24, that being on theorder of 16 to 20 minutes.

To maintain that minimum temperature, and yet avoid excessive heat andthe consequent oxidation of the scrap while the scrap is in the kiln 2,the bypass damper 59 of the system exhaust 10 is opened and closed inresponse to signals from its controller 86. In this regard, the bypassdamper 59, during normal operation of the kiln 2, remains partially openso that the airstream in the duct 56 that extends away from theafterburner 8 is directed both to the heat exchanger 12 and out thesystem exhaust 10; that is to say, it divides at the intersection of theducts 56 and 58. Should the temperature of the airstream where it isdischarged from the kiln 2 drop below a prescribed temperature, thecontroller 86 will cause the motor 60 to move the exhaust damper 59 sothat it further restricts the exhaust duct 58. This diverts more of theairstream to the heat exchanger 12 and thence into the duct 30 whichopens into the kiln 2. On the other hand, if the temperature of theairstream as it leaves the kiln 2 exceeds the prescribed temperature,the controller 86 causes the motor 60 to open the exhaust damper 59still further, so more of the airstream is diverted out of the systemexhaust 10 and less into the heat exchanger 12 and the kiln 2 which liesbeyond it. In short, the control loop 16 varies the mass flow of theairstream to maintain a prescribed temperature where the airstreamleaves the kiln 2.

The third control loop 18 insures that adequate oxygen exists within thecombustion chamber 40 of the afterburner 8 to consume all volatilescarried into the combustion chamber 40 by the airstream. It does this bemaintaining the oxygen content of the airstream downstream from theafterburner 8 within prescribed limits, such as 6%-8% by volume, this ofcourse being the oxygen content of the airstream where it is dischargedfrom the duct 30 into the kiln 2. The control loop 18 includes an oxygensensor 88 that is located along the duct 56, and a controller 90 whichoperates the motor 54 in the air line 48 that leads to the afterburner8. If the sensor 88 detects that the airstream is deficient, thecontroller 90 will cause the motor 54 to open the valve 50 still furtherso that it admits more heated air to the afterburner 8. On the otherhand, if the sensor 88 detects an excess of oxygen, the controller 90will cause the motor 54 to close the valve 50 somewhat and therebyfurther restrict the air line 48 leading to the afterburner 8.

In the operation of the system A, aluminum beverage cans or otheraluminum scrap covered with an organic coatings is introduced into theair lock 26 at the feed end of the kiln 2, whereupon it passes into thecylindrical chamber 24 of the kiln 2. The fragmented scrap, owing to therotation of the kiln 2, as well as to the inclination of the kiln 2 andflights, tumbles through the chamber 24, and in so doing migrates fromthe feed end to the discharge end, the residence time for any particularfragment being on the order of 16 to 20 minutes. At the discharge end,the fragmented scrap drops into the other air lock 28, through which itis removed from the system A.

During its residence time within the kiln 2, the fragmented scrapencounters the airstream which enters the kiln 2 at its discharge endand leaves at the feed end. Thus, the airstream flows in the directionopposite to that of scrap and the scrap reaches its highest temperaturejust as it drops into the air lock 28. Since the temperature of theairstream within the supply duct 30 never exceeds 1125° F, the scrapwithin the kiln 2 never exceeds that temperature--and that temperatureis below the melting temperature for the scrap, yet is above thetemperature at which the coating volatilizes. Moreover, the airstreamwithin the duct 30 has a reduced oxygen content, normally on the orderof 6% to 8%, and at this rarefied level of oxygen most coatings normallyfound on aluminum, whether they be lacquer or simply oils, will notignite, even at the highest temperature of the airstream within the kiln2. The coating does volatilize and enter the airstream, and any solidswhich remain simply become entrained in the airstream as particulatematter. Of course, as the airstream flows over the scrap within the kiln2, it, being hotter than the scrap, loses heat to the scrap and becomescooler.

The airstream flows through the discharge duct 32 to the cyclonecollector 4 where the particulates drop out and are collected. Even sothe volatilized components of the coating remain and flow on to theafterburner 8.

Within the afterburner 8, the airstream encounters substantially highertemperatures due to the presence of the flame produced by the burner 42.Moreover, at the entrance to the afterburner 8 the airstream acquires ahigher oxygen content due to the introduction of the air from the airline 48. The elevated temperature, together with the additional oxygen,provide an atmosphere suitable for combustion; that is, ignition of thevolatilized components of the coating. They are consumed and as a resultare converted primarily into carbon dioxide and water. The combustionleaves the airstream again deficient in oxygen--indeed, reduces itsoxygen content to the prescribed level of 6% to 8%.

Beyond the afterburner 8, some of the airstream is diverted to theatmosphere through the system exhaust 10, while the remainder passes onto the heat exchanger 12. Passing through the flow path 62 of the heatexchanger 12, the airstream loses heat to the cooling air which flowsthrough the other flow path 64 of the exchanger 12, and as aconsequence, the temperature of the airstream drops from about 1600° F.to 1125° F., the latter being the temperature at which it enters thekiln 2.

The entering temperature of 1125° F. is maintained by the first controlloop 14. If the temperature of the airstream at the entrance to the kiln2 becomes too high, the control loop 14 senses this and increases theflow of cooling air through the heat exchange to thereby increase heatextracted from the airstream at the heat exchanger 12. On the otherhand, if it drops, the control loop 14 reduces the flow of cooling airto thereby reduce the heat extracted from the airstream. In short, thecontrol loop 14 regulates the cooling air flowing through the flow path64 of the heat exchanger 12, and thereby controls the temperature of theairstream in the other flow path 62.

While the temperature of the airstream entering the kiln 2 remainssubstantially constant at about 1125° F., the mass flow of the airstreamdoes not. It varies to maintain a generally constant or uniformtemperature gradient within the kiln 2 (FIG. 2). Whereas, thetemperature of the airstream where it enters the kiln 2 is about 1125°F., the temperature where it leaves is about 450° F. To maintain thegradient, the controller 86 of the second control loop 16, by operatingthe motor 60 of the system exhaust 10, controls the amount of theairstream diverted to the atmosphere between the afterburner 8 and heatexchanger 12 and thus controls the mass of the airstream passing intothe kiln 2 at any given time. If the temperature of the airstream whereit leaves the kiln 2 is too low, the mass flow is increased by slightlyclosing the damper 59 of the system exhaust 10. On the other hand, ifthe temperature is too great, the damper 59 is opened slightly.

Were it not for the second control loop 16, and its ability to regulatethe mass flow of the airstream, within the kiln 2, conditions would varysubstantially within the cylindrical chamber 24 of the kiln 2, becauseit is virtually impossible to maintain any uniformity in the fragmentedscrap. First of all, the scrap does not pass uniformly through the kiln2, that is to say the mass of scrap within the kiln 2 will vary, indeedsubstantially, from time to time. Of course, the mass of scrap withinthe kiln 2, to a large measure, determines the amount of heat extractedfrom the airstream passing through the kiln 2; the greater the mass ofthe scrap, the more heat extracted. Aside from that, the scrap maycontain moisture, particularly if it constitutes expended beverage cans,and water, of course, requires considerable energy to convert to itsvapor phase. The amount of moisture may vary considerably, and thus theheat extracted from the airstream also depends on the amount of moisturethat is within the scrap in the kiln 2.

Thus, the temperature of the airstream at the discharge end of the kiln2, where the airstream enters the kiln 2, remains constant, at about1125° F., and likewise the temperature of the airstream at the feed endof the kiln 2, where the airstream leaves the kiln 2, likewise remainsconstant at about 450° F., irrespective of the mass of the scrap withinthe kiln 2 or the amount of moisture in that scrap (FIG. 2). The systemA responds to variations in the condition of the scrap by varying themass flow of the airstream through the kiln 2, that is, the mass flowpast any given point in the kiln 2 for a given unit of time. Between theinlet and outlet temperature the airstream experiences a gradualdecrease in temperature, that is it possesses a gradient. Along thegradient the scrap never quite reaches the temperature of the airstream,but always remains slightly below it. Indeed, the temperature of thescrap at any point within the kiln 2 varies slightly, that is it lieswithin an envelope of about 50° F., with the envelope being at atemperature slightly less than the temperature of the airstream at thatlocation in the kiln 2. Even at the discharge end of the kiln 2, wherethe temperature of the airstream is at its highest, the airstream is nothot enough to melt the scrap. Indeed, the airstream at that location inthe kiln 2 is about 100° F. less than the melting temperature ofaluminum and the actual temperature of the aluminum at that location isabout 100° F., lower.

While the temperature of the airstream exceeds the combustiontemperature of the volatile components in most coatings, the coatings donot ignite, because the control loop 18 senses the oxygen content of theairstream entering the kiln 2 and regulates it so that it remainsbetween 6% and 8%, which is below that required to sustain combustion.As a consequence, the volatile components merely volatilize and becomepart of the airstream, while the solid components drop off asparticulates which become entrained in the airstream. The cyclonecollector 4 thereafter extracts these solid components from theairstream before the airstream enters the recirculation fan 6 and theafterburner 8.

A modified system B (FIG. 3) likewise removes coatings from aluminumscrap and in many respects is quite similar to the system A. Indeed, thesystem B includes the kiln 2, the cyclone collector 4, the fan 6, theafterburner 8, the system exhaust 10 and the heat exchanger 12, as wellas the ducts 30, 32, 36 and 56 which connect them. It also has the ventduct 70 and damper 72 and the motor 74 that controls the damper 72. Inaddition, it has a bypass duct 100 which extends between the supply duct66 and the discharge duct 68 to thereby bypass the flow path 64 of theheat exchanger 12. The bypass duct 100 contains a damper 102 which isoperated by a motor 104.

While the modified system B retains the control loop 18 and its oxygensensor 88 and controller 90, it does not have the control loops 14 and16. Instead it has several different control loops 106, 108, 110, 112and 114.

The control loop 106 includes a temperature sensor 120 which detects thetemperature of the air in the duct 32, which is of course, thetemperature of the airstream where it discharges from the kiln 2. Thesensor 120 is connected to a controller 122 which controls the speed ofthe motor 37 for the fan 6. When the kiln 2 experiences a greater demandfor heat, such as by reason of a greater mass of scrap or increasedmoisture content, the temperature of the air in the duct 32 drops. Thesensor 120 detects this drop in temperature, and the controller 122,reacts to increase the speed of the motor 37. The fan 6 in turn forcesmore air through the ducts 30, 38 and 56 so that a greater volume ofair--and likewise mass of air--flows into the kiln 2 per unit of time,and of course this greater mass has the capacity to deliver more heat tothe aluminum scrap. The temperature of the scrap increases and so doesthe temperature of the airstream leaving the kiln 2 by way of the duct32. Of course, the reverse also occurs. If the temperature of theairstream in the duct 32 increases, the speed of the fan motor 37decreases and thereby reduces the mass flow of air.

The control loop 108 operates the system exhaust 10 and as such controlsthe motor 60 for the damper 59 in the exhaust duct 58. It seeks tomaintain constant pressure within the duct 56, as well as within theduct 30 and the cylindrical chamber 24 of the kiln 2, with that pressurebeing only slightly above atmospheric, for the kiln 2 operates moreefficiently at a slightly elevated pressure. To this end, the loop 108includes a pressure sensor 124 which detects the pressure in the duct 56at the discharge from the afterburner 8, and a controller 126 which isconnected to the sensor 124 and operates the motor 60 of the exhaustdamper 59 in response to the pressure detected in the duct 56. If thatpressure increases, the controller 126 causes the motor 60 to open thedamper 59 and release more air through the system exhaust, 10 so thatexcessive pressure does not develop within the ducts 30 and 56.Conversely, if the pressure drops, the controller 126 causes the motor60 to close the damper 59 so that a prescribed minimum pressure exists.The optimum pressure provides good thermal efficiency without fugativeemissions past the seals of the kiln 2.

The control loop 110 controls the temperature of the air entering thekiln 2. It includes a temperature sensor 128 which is located in theduct 30 and a controller 130 which is connected to the sensor 128. Thecontroller 130 in turn is connected to and operates the motor 104 forthe damper 102 in the bypass duct 100. Should the temperature of theairstream in the duct 30 drop below the prescribed value, which isnormally 1125° F., the controller 130 causes the motor 104 to open thedamper 102 so that less cooling air from the fan 65 passes through theflow path 64 of the heat exchanges 12. As a consequence, less heat isextracted from the airstream flowing through the other flow path 62, andsince the flow path 62 opens into the duct 30, the temperature of theairstream in the duct 30 rises. Conversely, if the temperature in theduct 30 rises, the controller 130 causes the motor 104 to close thedamper 102, and this diverts more cooling air through the flow path 64of the heat exchanger 12 to thereby lower the temperature of the airflowing into the duct 30.

The control loop 112 insures an adequate supply of cooling air for theflow path 64 of the heat exchanger 12. To this end it monitors thetemperature of the air that flows through the discharge duct 68 to theafterburner 8. If that air becomes too hot, thus indicating aninsufficient flow, it causes the motor 74 to open the damper 72 so thatsome of the cooling air is discharged to the atmosphere; and this inturn allows a greater volume to flow through the flow path 64 of theheat exchanger 12. In this regard, sometimes the airstream entering thecombustion chamber 40 of the afterburner 8 contains so many volatilesthat little, if any, heat is required from the burner 42 to consume themand maintain adequate temperatures in the combustion chamber 40. Thus,the burner 42 draws little, if any, combustion air from the dischargeduct 68, and this would severly restrict the flow of cooling air throughthe path 64 of the heat exchanger 12, were it not for the discharge ofair through the vent duct 70 and damper 72. The control loop 112 seeksto maintain the temperature of the combustion air that flows through theduct 68 to the burner 42 at about 1100° F. It includes a temperaturesensor 132 located in the duct 68 downstream from the vent duct 70 and acontroller 134 which responds to the sensor 132 and operates the motor74 for the damper 72 in the vent duct 70.

The control loop 114 maintains a minimum temperature within theafterburner 8, and that minimum should be high enough to insure that allvolatiles derived from the scrap are consumed. It includes a temperaturesensor 136 in the combustion chamber 40 of the afterburner 8 and acontroller 138 which is connected to and responds to the sensor 136. Thecontroller 138 is connected to the motor 52 which operates the valves 45and 47 that admit fuel and combustion air to the burner 42 of theafterburner 8. If the temperature within the chamber 40 falls below theprescribed minimum, which should be on the order of 1600° F. for scrapcomposed primarily of beverage cans, the controller 138 causes the motor52 to open the valves 45 and 47 still further and thus increase the heatdelivered by the burner 42. The temperature within the combustionchamber 40 therefore rises.

In operation, the system B accepts used aluminum beverage cans or otheraluminum scrap at its air lock 26 and conveys the scrap through the kiln2, from which it is discharged at the air lock 28, all as in the systemA. Moreover, the system B creates a rarefied atmosphere of 6% to 8%oxygen in the kiln 2, and at the entrance to the kiln 2 maintains thetemperature of the airstream at a prescribed temperature below themelting point of aluminum, preferably at 1125° F. The mass flow of air,however, varies to accommodate variances in the mass and constituency ofthe scrap within the kiln 2. Due to the counterflow construction of thekiln 2, the air possesses its greatest temperature where it enters thekiln 2 at the duct 30, whereas the scrap achieves its greatesttemperature where it leaves at the air lock 28. Of course, the heatedair volatilizes the coatings, and the volatile components enter theairstream as do particulates.

The cyclone collector 4 separates the particulates from the airstream,but the volatile components pass on to the afterburner 8. Here they areconsumed in the combustion chamber 40 where the temperature and oxygencontent are high enough to sustain combustion of them.

The combustion within the chamber 40 of the afterburner 8 wouldsubstantially increase the pressure within the duct 56 leading from theafterburner 8, were it not for the system exhaust 10. Indeed, thecontroller 126 and the pressure sensor 124 monitor the pressure withinthe duct 56 and, by controlling the motor 60 for the exhaust damper 59,open and close the damper 59 to maintain essentially constant pressurein the ducts 30 and 56, and in the cylindrical chamber 40 of the kiln 2as well. That pressure is slightly above atmospheric, which insuresefficiency, but is not so high as to cause the air to escape throughseals in the kiln 2.

The temperature of the rarefied air entering the cylindrical chamber 24of the kiln 2 remains essentially constant at 1125° F. or at any othertemperature that is selected. In this regard, the sensor 128 in the duct30 detects the temperature of the airstream flowing in the duct 30 andis of course monitored by the controller 130. Should the temperature ofthe airstream drop, the controller 130 causes the motor 104 of thebypass damper 102 to open the damper 102 so that less cooling air flowsthrough the path 64 of the heat exchanger 12. As a consequence, lessheat is extracted from the airstream flowing through the other path 62,and that of course is the air which passes into the duct 30. Thus, thetemperature of the airstream in the duct 30 increases. The converselikewise holds true; that is, when the temperature of the airstream inthe duct 30 increases, the damper 102 closes to direct more cooling airto the heat exchanger 12.

While the control loop 110 seeks to maintain the temperature of the airat the entrance to the kiln 2 constant, the control loop 106 prevents anexcessive drop in the air temperature as the air passes through the kiln2, or in other words maintains a prescribed gradient within the kiln 2(FIG. 2). As such, it seeks to maintain a generally constant temperaturefor the air that leaves the kiln 2 through the duct 32. Thus, should thescrap introduced into the kiln 2 place greater demands on the airstream,such as by reason of greater mass or increased moisture, more heat willbe extracted from the airstream and the temperature within the dischargeduct 32 will drop. The sensor 120 and controller 122 detect this, andthe latter causes the motor 37 of the fan 6 to operate at greater speedand thus force more air through the ducts 30, 56 and 30 and ultimatelyinto the kiln 2, yet the temperature of the air at the entrance of thekiln 2, that is in the duct 30, remains the same. The increased massflow compensates for the greater demands imposed on the kiln 2, and as aconsequence, the temperature of the air leaving the kiln 2 through theduct 32 rises. The converse likewise occurs, that is when thetemperature in the duct 32 becomes too high, the controller 122 reducesthe speed of the fan motor 37 so that the fan 6 forces less air throughthe kiln 2.

If the demands of the afterburner 8 for combustion air do not meet thecooling needs of the heat exchanger 12, the temperature of thecombustion air in the duct 68 will rise. The sensor 132 will detect thisand the controller 134 will cause the motor 74 to open the vent damper72 so that more air passes through the flow path 64 of the heatexchanger 12 to the airstream as it passes through that flow path.

This invention is intended to cover all changes and modifications of theexample of the invention herein chosen for purposes of the disclosurewhich do not constitute departures from the spirit and scope of theinvention.

What is claimed is:
 1. A process for removing from aluminum scrapcoatings having volatile components, said process comprising: passingthe aluminum scrap through a kiln, the interior of which is isolatedfrom the surrounding atmosphere; directing an airstream through theinterior of the kiln; heating the airstream, before it enters the kiln,to a temperature in excess of that required to volatilize the coatingand contemporaneously reducing the oxygen content of the airstream tobelow that necessary for supporting combustion of the coating within thekiln, so that the airstream within the kiln will volatilize the volatilecomponents of the coating, but will not ignite those components;extracting heat from the airstream before it enters the kiln to maintainthe temperature of the airstream where it enters the kiln substantiallyconstant at a magnitude high enough to volatilize the coating, yet belowthe melting point of aluminum; and varying the mass flow of theairstream through the kiln to maintain the temperature of the airstreamwhere it leaves the kiln substantially constant, whereby the airstreamaccommodates variations in the mass flow of the scrap and the nature ofthe scrap within the kiln.
 2. The process according to claim 1 whereinthe step of varying the mass flow includes diverting a portion of theairstream after it is heated, and controlling the portion so diverted.3. The process according to claim 1 wherein the step of directing theairstream into through a kiln includes forcing it into the kiln with afan; and the step of varying the mass flow includes varying the speed ofthe fan.
 4. A process for removing coatings having volatile componentsfrom aluminum scrap, said process comprising: generating an airstream;directing the airstream into the interior of a kiln; passing through thekiln aluminum scrap having a coating provided with volatile components;heating the airstream before it enters the kiln; maintaining the oxygencontent of the airstream at less than that of ambient air; where theairstream enters the kiln, maintaining the temperature of the airstreamsubstantially constant and high enough to volatilize the coating withoutmelting the aluminum, wherein the coating volatilizes in the kiln andenters the airstream; and without substantially altering the oxygencontent of the airstream, varying the mass flow of the airstream throughthe kiln to maintain the temperature of the airstream where it leavesthe kiln substantially constant, whereby the airstream accommodatesvariations in the mass flow of the scrap through the kiln and variationsin the nature of the scrap within the kiln.
 5. The process according toclaim 4 wherein the step of maintaining the temperature of the airstreamsubstantially constant where it enters the kiln includes extracting heatfrom the airstream before it enters the kiln and after it is heated. 6.The process according to claim 5 wherein the step of generating theairstream includes directing the airstream through a fan.
 7. The processaccording to claim 6 wherein the step of varying the mass flow of theairstream includes varying the speed of the fan.
 8. The processaccording to claim 5 wherein the step of heating the airstream includessupplying combustion air to the airstream and igniting a combustiblesubstance within the airstream.
 9. The process according to claim 8 andfurther comprising using the heat extracted from the airstream to heatthe combustion air.
 10. The process according to claim 4 and furthercomprising isolating the kiln from ambient air so that its oxygencontent remains less than that of ambient air.